Process for purifying gases



July 29; i941. F, M; PYZEL 2,251,000

PROCESS FOR PU'RIFYING GAsBs 4 l Filed oct. 2, 1959 Wa/r Sejoarafors I I /Chreref Compressor Sary@ r//f 22 Patented July. 29, 1941 UNITED fsTA-'r Frederic vMarinus Pyzel, Piedmont, Calif.,Y as'- .signor to Shell Development Company, San Francisco, Calif., a corporation of Delaware ppueauqn october a, 1039, serial Nolzsasss V (c1. zs`a 3 Claims.

The present invention relates to the removal of impurities from gases, and more particularly to the removal 9i carbon monoxide from hydrogen to be used in chemical synthesis under pressure.

It is a well' known fact that oxygen,` water, carbon monoxide, carbon dioxide and other impurities exert a harmful ,effect upon catalytic masses -used for ammonia synthesis and in the U hydrogenation of hydrocarbons, and that'these impurities, even in small quantities, eventually cause an impairment in the eiiectiveness of the catalyst when present in the make-up gas.

A number of processes have already been pro--A posed withaview to removing these harmful impurities in the gases, whereby the effectiveness of the catalyst employed in chemical synthesis is prolonged, as, for example, carbon monoxide may be removed by catalytic hydrogenation to methane, according tothe reaction:

' Heretofore it has not been considered economically feasible to remove relatively large quantities of carbon monoxide from gases by convertsion to methane due to the rapid poisoning` of the catalyst used for such conversion. l

An object of this invention is to provide a improved method for removing carbonfmonoxide by conversion to methane. f

The temperature at which the reaction of carbon monoxide to methane initiates will vary with thetype of converter and the concentration of carbon monoxide in the gas to be treated. In adiabatic converters a temperature oi about 240 C. is necessary when the incoming gas contains approximately 0.7 volume per cent of carbon y monoxide. With 1.0 volume per centv an initiating temperature of 230 C. is required, while at 1.5 volume per cent concentration an initiating temperature of 210 C. suffices.

In an adiabatic converter the temperature of the gas increases about 70 C. for every one volume per cent cf carbon monoxide converted.v

that the presenceof water vapor in the converter will cause a temporary deactivation of the catalyst. `It has been found that a water vapor content of as much as 5 volme percent will have ^an extremely deleterious eilect upon the catalyst,

' l-volume per cent or less, preferably 0.7 volume e yper cent, being. the approximate upper limit at rwhich the catalytic reaction may be eiilcientl carried out. Y

The present invention contemplates a system whereby the concentration oi carbon monoxide and water vapor in the gas entering the converter is maintained at an optimum ilgure, to

insure both complete puriilcation and long catalyst life.

Referring to the accompanying schematic ilow r diagram,l a 'gas-holder I containing carbon monoxide contaminated gases 'including hydrogen, is provided with a conduit 2 leading through valvev 3 to the rst stage oi a two-stage compressor 4. vFrom the discharge side of the: iirst stage of the compressor 4 aF conduit V5 leads through a cooler 8 and water separator 1 to the second stage oix the compressor 4. .Conduit 8 leads from the second stage of the compressor 4 to an after-cooler 9, water separator `II) and wash towersil and I2. Conduit I3. leads from wash tower I2 to a preheater I4 through valve I5. Conduit I8 leads through valve II to a converter I8. Conduit I9 leads from the converterA I8 to a surge tank 20, passing through a cooler 2 I. Surge tank 20is provided with a water dropout at 22, a conduit 23 leading through valve 24 and a conduit 25 leading through a vlverzs td. a. circulatory compressor 21 and thence to valve 28 and conduit I3.

In operation the system operates as a continuous circulatory process. Contaminated gas is passed under pressure from the gas-holder I throughconduit 2, valve 3, the iirst stage of compressor 4, conduit I, cooler 6, water separator 1,` the second stage `of compressor 4, conduit 8, aitercooler 9, water separator I0`and wash towers II and I2. In this manner, water vapor in the raw gas is removed during the compression and cooling steps and other impurities exclusive of carbon monoxide, for example hydrogen sulfide and carbon dioxide, removed in the wash towers. a

rGas containing only carbon monoxide as an impurity is then passed'through conduit- 13 and valve I5 to preheater I4 and thence through conduit I8 -and valve Il to converterA I8, The exiting gas from the converter passes through conduit a i'actor that mustbeconsidered. lIt isknown 553 I9-and cooler 2| to surge tank 20. Water vapor resulting from the reaction and condensed in cooler 2| is dropped out at 22, purlned gas is passed through conduit 22 and valve 24 to utiiizstion and part of the purified gas, the quantity depending upon the CO concentration of thel contaminated gas entering the converter, is recirculated through conduit 25, valve 2l, compressor 21, and valve 28 to conduit Il where it is mixed with the incoming unpuried gas.

As a specific example, assuming that it is desired to remove carbon monoxide by conversion to methane in the presence of nicket catalyst from hydrogen produced by thermal decomposition of hydrocarbons and containing 3 volume,

per cent of carbon monoxide, it having been previously determined experimentally that :0.7 volume per cent carbon monoxide is the optimum amount that may be removed with relatively little damage to a nickel catalyst. then the contaminated gas will be passed to the converter after being compressed to a pressure of six atmospheres, and having its water content reduced to 0.7% (namely, saturation at atmospheric temperature and a pressure of six atmospheres) by means of the coolers 6 and 9 and water separators 1 and l0. In this manner, the water con-f tent present, which is known to rimpair the edlciency of the catalyst, is reduced to a safe value. The entering gases are brought up to the initial reaction temperature of approximately 240 C. in the preheater I4.

At the same time, carbon monoxide-free hydrogen is supplied to the converter from the surge tank through the circulating compressor and prefheater. By controlling valves 26, 28, and l5, the ratio of puried hydrogen to contaminated hydrogen fed to the converter in this case will be maintained at about 4:1, in order to reduce the carbon monoxide content of the gas entering the converter from 3 volume per cent to 0.7 volume per cent.

It will thus be seen that we have a system in which the water content of the raw gas'is reduced to a safe gure through compression and cooling (the gas, however, being saturated with water vapor at the increased pressure). VThe high CO content is reduced by dilution with gas previously treated (but saturated with water vapor at the pressure of the system). The mixed gas stream is passed over the catalyst whereby water vapor is formed. The gas stream already whereas an attempt to remove 3 volume per cent carbon monoxide directly would result in complete catalyst failure `within a matter of hours.

Although the example cited illustrates carbon monoxide removal from hydrogen in an adiabatic converter in the presence of a nickel catalyst, it is to be understood that this is by way of illustration only. Other converter types, as for example those of the externally heated type and other catalysts, as for example cobalt and palladium, are equally susceptible to the present improved system of operation.

I claim as my invention:

1. In a continuous process for removing carbon 4 monoxide from hydrogen containing gases' contaminated with more than 0.7 per cent by volume of carbon monoxide by catalytic conversion to methane, the steps comprising passing an entering stream oi carbon monoxide contaminated hydrogen containing gases to a reaction zone containing a nickel catalyst, reacting said carbon monoxide to produce methane and water, continuously withdrawing gases from said reaction zone, separating the water content Yrom said withdrawn gases, and continuously recycling a portion oi! the water free and carbon monoxide being saturated with water vapor, this excessy free withdrawn gases to said entering stream in quantities sumcient to maintain the carbon monoxide concentration in said entering stream at a value not exceeding approximately 0.7 volurne per cent.

2. In a continuous process oi' removing carbon monoxide from hydrogen containing gases, having more than 0.7 per cent by volume of carbon monoxide, by catalytic conversion to methane, wherein said gases are passed through a reaction zone containing a catalyst mass containing a metal selected from the .group consisting of nickel, cobalt and palladium, the steps of separating water vapor from the gases leaving the reaction zone, and recycling a suillcient quantity of the substantially water free exit gases to said reaction zone to maintain the carbon monoxide content of the gas in the reaction zdne at less than 0.7 per cent byvolume.

3. In a continuous process for removing carbon monoxide from hydrogen containing gases having more than 0.7 per cent by volume of carbon monoxide, the steps comprising passing a stream of hydrogen y,containing gas through a reaction zone containing a nickel catalyst, continuously withdrawing a carbon monoxide free gas from the exit of said reaction zone, and adjusting the recycle of sumcient of said carbon monoxide free gas to the entrance of said reaction zone so that the gas passing to the nickel catalyst in said re- 

